Two photomachinable (“PM”) glass compositions, known under the name Fotoform™ (Corning Incorporated) and Foturan® (Schott Glass). By photomachinable is meant that after the glass is covered with a patterned photomask and exposed to actinic radiation, preferably UV light, followed specified thermal treatment, the resulting glass-crystal composite material, in the area not covered by the photomask (that is, the exposed or pattern areas) is preferentially soluble in hydrofluoric acid relative to the unexposed glass. This permits one to make patterns of holes and cavities utilizing a conventional photolithographic process. There were numerous commercial applications that utilized this patented material where the refractive nature of the material along with chemical durability, non-magnetic and electrical insulating properties made it unique. However, one property that limited its application was a relatively high coefficient of thermal expansion (“CTE”). The Corning Fotoform™ material has a CTE of 8.4×10−6/° C. and the Schott Foturan® material has a published CTE of 8.6×10−6/° C.
Photomachinable glasses are based on photosensitively opacifiable glasses (also called “opal glasses”) having a lithium disilicate and/or lithium metasilicate (also called lithium monosilicate) crystallite phase. Photosensitively opacifiable glasses which do not contain lithium are described U.S. Pat. Nos. 2,515,939, 2,515,941, 2,559,805, 2,651,145, and 2,651,146. Photosensitively opacifiable glasses that do contain lithium disilicate and/or lithium metasilicate are described in U.S. Pat. Nos. 2,515,940, 2,628,160 and 2,684,911. Opal glasses with patterns or designs are formed by masking an appropriate glass composition with a patterned mask, subjecting it to radiation in a photolithographic process and a subsequent heat treatment as briefly described in the preceding paragraph and in detail in U.S. Pat. Nos. 2,515,939, 2,515,941, 2,559,805, 2,651,145, and 2,651,146. In addition, U.S. Pat. Nos. 2,628,160 and 2,684,911 describe the chemical machining of an opalized glass (photosensitive) containing lithium disilicate and/or lithium metasilicate, for example, by use of a dilute hydrofluoric acid solution. The radiation used in the foregoing patents was supplied using ultraviolet emitting lamps, for example, Hg or Hg—Xe arc lamps, which flooded the entire surface (lithographic mask and exposed glass) of the article being irradiated with less than 400 nm radiation. U.S. Pat. No. 2,515,939 indicates that generally the time and intensity of the radiation necessary to produce the desired effect was determined by trial. U.S. Pat. No. 2,515,941 indicates that for a given composition different colors can be developed in the glass using different radiation exposure times.
While the properties of the known PM glasses have enabled a number of uses, it is highly desirable to have a PM glass that has a lower CTE value to extend the use of photomachinable glasses. In particular, it is desirable to have a PM glass having a CTE that more closely matches that of silicon (CTE approximately 2.6×10−6/° C.) because this feature will permit a wider range of new possible applications.